The invention relates generally to signal propagation delay estimation, and more particularly to signal propagation delay estimation using frequency domain techniques.
Emergency 911 service is rapidly becoming important in today""s society. One of the compelling reasons for using the existing landline emergency 911 system is the ability to trace the caller""s location. Using databases in the telephone network switches, the caller""s location is determined and made available to the emergency services. In the event the caller is unable to inform the operator of their location, the ability to trace the call is invaluable.
The explosive growth of mobile phones, however, causes complications for emergency 911 services. While mobile users may call the 911 operator just as they would using a landline phone, there is currently no ability to trace the location of the mobile caller. The emergency 911 operator can currently only trace the mobile call to the base station closest to the mobile caller is using.
Mobile systems with the ability to locate mobile callers are known as wireless enhanced 911 or E911 systems. One known approach to determine a mobile caller""s location involves using an improved handset. The improved handset may incorporate a global positioning system (GPS) receiver to determine the mobile caller""s location and then transmit the location to the emergency 911 operator. Another improved-handset may use signals from multiple base stations to determine the mobile caller""s location. These handset improvements, however, involve improved handset circuitry that increases the cost of the handsets. Further, the extra circuitry requires extra battery power. Moreover, deployment of the improvement takes time since it depends on the users upgrading their handsets.
Another approach would not modify the handsets, thereby avoiding the drawbacks stated above. The so-called network approach involves modifying the base stations. One such approach involves angle of arrival techniques using improved antenna arrays at the base station. Another approach uses a rough determination of the mobile""s transmission characteristics and compares these to a large database of the surrounding environment to determine a rough idea of the mobile""s location. In the first approach, however, new antennas must be installed which may be expensive. In the second approach, maintaining the database is difficult since the environment changes readily.
There is, however, a network based approach that involves a minimum amount of hardware upgrade and does not require knowledge of the surrounding environment. The approach is known as Time Difference Of Arrival or TDOA. TDOA involves measuring the Time Of Arrive (TOA) of the mobile""s signal at multiple base stations. The TOAs are then sent to the Central Office and subtracted to get a measurements of TDOA between base stations. These TDOA parameters are then used to locate the mobile""s position using standard hyperbolic locating techniques.
Known TDOA systems estimate the TOA by correlating the received signal with a known reference. The accuracy of the TOA estimation is limited by the resolution of the correlation. Furthermore, multipath and interference may degrade the performance of this technique.
Clearly, there is a need to accurately estimate the Time Of Arrival for mobile signals in a multipath environment.
The invention is directed to an apparatus and method for accurately estimating the time of arrival of a received signal in a multipath environment.
An aspect of the invention is a method for estimating a time delay. The method includes the following steps. First, estimating a channel spectrum from a received signal and a known reference signal. Second, extracting a virtual array structure from the estimated channel spectrum. Third, determining the signal time delay using the virtual array structure.
Another aspect of the invention is a system for estimating a time delay. The system includes a channel spectrum estimator, means for extracting a virtual array structure and means for determining the time delay. The channel spectrum estimator is used for estimating a channel spectrum from a received signal and a known reference signal. The means for extracting a virtual array structure uses the estimated channel spectrum. The means for determining the time delay uses the virtual array structure.
An advantage of the invention is the ability to accurately determine a time delay of a received signal in a multipath environment. In many environments multipath effects can be significant, thus the ability to accurately determine the time delay of a received signal can be difficult using existing systems.
Other aspects and advantages of the invention, as well as the structure and operation of various embodiments of the invention, will become apparent to those ordinarily skilled in the art upon review of the following description of the invention in conjunction with the accompanying drawings.